Theories of the universe add up to nought

December 22, 2002 — 11.00am

Physicists tend to be simple folk. Contrary to popular mythology, they like to explain the world in the simplest possible terms. For the past half millennium, Occam's Razor has been their guiding light: why suppose the existence of anything more than the absolutely necessary minimum? In keeping with this parsimonious spirit, most theoretical physicists have believed that, ultimately, there must be just one possible universe, the physical manifestation of a set of laws so compelling that no other option would be viable. One universe, one theory, one defining way.

It was a lovely idea, but increasingly it seems a fantasy. In recent years, theory and experiment are leading to the conclusion that, far from being the only option, our universe may be just one among an almost infinite array of possible worlds. It may be that ours is simply one member of a vast cosmological swarm.

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Several paths seem to be leading in this direction. The most notable is string theory, which is the leading contender for a so-called theory of everything. Many physicists are convinced that some version of string theory will prove to be the final description of all physical reality, a unification under one mathematical umbrella of matter, force, space and time.

As the name so charmingly implies, string theory proposes that, at its core, the universe is composed of minute strings. To get a sense of what this means, imagine a subatomic particle as a tiny point; now further imagine that, as you look closer, this point turns out to be a tiny closed loop, not a simple point but a minute circle. According to string theory, everything in the universe can be explained in terms of these microscopic loops or strings.

The trouble is that there are many versions of the string theory equations. Making matters worse, each version allows for many solutions. Physicist Joseph Polchinski has recently estimated that there may be as many as 10 to the power of 60 solutions to these equations, and every solution represents a different possible universe. That's a million billion billion billion billion billion billion potential universes. To put this into perspective, there are about 400 billion stars in our galaxy.

Indeed, the number of universes Polchinski is suggesting exceeds the number of stars in our entire universe by many orders of magnitude.

Figures like this are difficult enough to grasp when talking about individual stars, but when talking about entire universes they are beyond comprehension.

Speaking by phone from his office at the Institute for Theoretical Physics at the University of California in Santa Barbara, Polchinski seems somewhat overwhelmed by his statistic, and hastens to say that some of the solutions may not turn out to be viable.

The idea that there might be more than one universe actually dates back to early last century when Albert Einstein proposed his General Theory of Relativity. General relativity describes the large-scale structure of the cosmos and any theory of everything will have to incorporate its insights.

Like string theory, general relativity is a set of equations that admits many solutions - and, again, every solution describes a different possible universe. One of the major problems in cosmology over the past century has been trying to determine which version of the Einsteinian solution fits our particular universe.

Many physicists had hoped that, eventually, one solution would emerge as a natural winner, but with the advent of string theory the problem has only compounded. Today, most physicists admit they are suffering a bad case of cosmological overload.

Which one of these potential universes do we actually live in? Polchinski says it is not clear if there will ever be enough data to sift through all the possibilities. He says, however, that understanding our universe is not a lost cause. It is just that it might simply take physicists a very long time to fathom it out. He notes that physics itself is a continually evolving science that is regularly invigorated with radical new ideas and mathematical tools.

String theory itself is a prime example, having been invented only about 20 years ago. In the next 50 years, Polchinski expects there to be at least two conceptual revolutions that should help to sort through the morass.

Polchinski stresses that the very crisis he and his colleagues are facing is a measure of how successful physics has been. Over the past four centuries, physicists have come to understand the universe at every scale from the subatomic to the intergalactic. Although there are many phenomena left to explore, it seems that, overall, physicists do now have a fairly comprehensive picture of how the world works.

Now that in some sense the universe we live in has been figured out, Polchinski says, we are confronted with the question of what framework explains that universe. And "why" questions are always much harder to answer than "how" questions.

Whatever type of universe ours turns out to be, general relativity and string theory suggest that, in principle, many kinds of universes might be possible. Do any of these other worlds actually exist? For Stanford University physicist Andrei Linde, other universes are not only possible, they are inevitable. An effusive Russian bear of a man, brimming with enthusiasm, Linde has developed a theory he calls eternal inflation, which proposes an infinite bubbling sea of universes, each as real and concrete as our own.

In Linde's theory, each universe is a unique bubble of space and time equipped with its own laws of physics and its own cosmic history. These other universes may differ wildly from our own, possessing different kinds of matter, different kinds of forces, even different numbers of dimensions.

In his theory, all universes spring into being from an enigmatic sea of potential that physicists call the quantum vacuum. Pre-dating matter and space, the quantum vacuum is a mathematical attempt to describe what happens before the beginning of time itself.

Present theory suggest our universe emerges from this vacuum as an infinitesimal blip that rapidly inflates, like a balloon. Linde ponders, if such a process could happen once, whether it could happen many times. Or, as physicist Craig Hogan recently put it, once it has been discovered it is easy to make a universe out of an ounce of vacuum, why not make a bunch of them?

Lee Smolin, a quantum gravity authority at Canada's Perimeter Institute, proposes a further mechanism for generating universes. In his theory, baby universes continually bud off older universes anywhere there is a black hole. It is a bit like coral spawning, with each universe constantly spawning tiny new bubbles of spacetime, each of which then undergoes its own Big Bang to become a fully fledged cosmos. As Smolin sees it, each generation of universes undergoes a subtle mutation of its physical laws so that the offspring universes are slightly different from their parents.

This evolutionary process gives rise to a vast Darwinian landscape of universes, an infinite ecology in which our humble spacetime bubble is just one member of an endlessly variable chain of cosmic procreation. If Linde and Smolin are correct, almost any kind of universe that is theoretically possible will be realised somewhere. On that extravagant note, I wish you all a happy and abundant Christmas season.